Optical sensors (optrodes) for measuring pH are well known. Certain aromatic organic compounds (like phenolphthalein) change color with pH and can be immobilized on solid supports to form “pH paper.” These visual indicators are easy to use, but do not provide a quantitative reading. The color changes can be difficult to distinguish accurately, and can be masked by colored analyte. Fluorescent indicators have also been used as optical sensors. pH Sensitive fluorescent dyes can be immobilized on solid supports and generally are more sensitive in comparison to the simple color changing (absorbance or reflectance based) indicators. The improved sensitivity of fluorescent indicators allows the solid support to be miniaturized, and this has been used to advantage in development of fiber optic sensor devices for measuring pH, CO2, and O2 parameters in blood.
A specific need in the medical industry exists for accurate pH measurement of blood. The pH of blood, or other bodily fluids (pleural effusions) can be associated with certain physiologic responses associated with pathology. Blood gas analyzers are common critical care instruments. Depending on storage conditions, the pH of separated blood components (plasma, platelets) can change rapidly due to off-gassing of dissolved CO2 from the enriched venous blood that is collected from a donor. Platelets in particular are metabolically active, and generate lactic acid during storage at 20-22° C. European quality guidelines for platelets prepared by the “buffycoat method” require pH of stored platelets to be pH 6.8-7.4 at 37° C. (7.0-7.6 at 22° C.).
Seminaphthofluorescein (SNAFL) compounds and the related seminaphthorhodafluor (SNARF) compounds are commercially available ratiometric fluors (Molecular Probes, Inc., Eugene, Oreg.; see, for example, U.S. Pat. No. 4,945,171) and their synthesis and spectral properties have been described. These compounds have advantages including long wavelength absorbance that can be efficiently excited with LED light sources. Relevant acid/base equilibria and associated spectral properties are shown below.
Deprotonation of the naphthol structure of SNAFL dyes gives a naphtholate molecule with longer wavelength fluorescence emission. The pKa is the pH value where the two molecular species form in equal amounts. SNAFL compounds with reactive linker groups that allow their conjugation to other molecules of interest are also commercially available.
Various methods have been used to immobilize “ratiometric” dyes to solid supports for use in fiber optic pH detectors. Carboxynaphthofluorescein (CNF) has been conjugated to aminoethyl-cellulose and this material was glued to polyester (Mylar) films to make sensing membranes for optrodes. The pKa of this material was 7.41, slightly lower than the free CNF (pKa 7.62). The use of tetraethoxysilane to trap CNF in a sol-gel glass that was formed on glass cover slips has also been reported. The pKa of this material was 7.46. A 9-chloro substituted SNAFL analog (SNAFL-2) has been reacted with polyvinylamine and the residual amino groups were crosslinked with a photocrosslinker to form a gel-like coating on acrylic fibers. The pKa of this fiber-optic sensor was 7.14, significantly lower than the published pKa of the free SNAFL compound (pKa ˜7.7). This shows that molecular environment and linker structure surrounding the immobilized dye can alter the performance of a pH detector.
Despite the advances made in the detection of pH noted above, there exists a need for improved methods and devices for measuring pH. The present invention seeks to fulfill this need and provides further related advantages.